Mulata is a term used in Spanish to describe a woman of mixed European and African ancestry. While the word's origins are complex, in the context of this song and Caribbean culture, it's used to describe a beautiful and captivating woman.
Jarabe de Palo sings, "Coral negro de La Habana, tremendísima mulata" (Black coral of Havana, a most tremendous 'mulata'). The word paints a vivid picture of the song's mysterious subject, 'La Flaca', celebrating her unique beauty and heritage.
This song is a love letter to a woman the singer met in Havana, Cuba. He calls her "La Flaca" (The Skinny Girl) and describes her as "Cien libras de piel y hueso / Cuarenta kilos de salsa" (One hundred pounds of skin and bone / Forty kilos of salsa). She might be poor, sleeping during the day to "engaña el hambre" (trick the hunger), but at night she comes alive, dancing and drinking in the local taverns. The singer is absolutely obsessed with her, saying she has "dos soles que sin palabras hablan" (two suns [for eyes] that speak without words).
This isn't just a crush; he's completely head over heels. The whole chorus is him repeating how he would give anything for just one kiss from her: "Por un beso de La Flaca / Daría lo que fuera" (For a kiss from The Skinny Girl / I would give anything). He's driven mad by his memories of her touch, confessing that this woman "a mí me tiene loquito" (has me going crazy). It's a song about total, consuming infatuation with a fascinating and unforgettable person.